Threaded fastener pairs (e.g., a bolt and a nut, a stud and a tapped hole, etc.) used in assembly of parts are intended to produce a clamp load that can be sustained when the assembled parts are subjected to service loads. The clamp loads produced during an assembly must be less than a maximum value in order to avoid damaging the parts. In most cases, the fasteners are the weakest elements. The maximum clamp load in such cases is the yield strength in the threaded areas, usually expressed in bolt tension. There are also applications where the clamped pieces have limited strength. The maximum value is then specified based on the characteristics of the clamped pieces. The clamp load must also be greater than a minimum value to prevent failure in service. The assembly process, from a clamp load point of view, therefore becomes a compromise between field failure (i.e., excessively low bolt tension) and assembly failure (i.e., excessively high bolt tension). The degree of compromise is dependent on the precision of the process of controlling the clamped load. The cost of imprecision is a bulky design and/or diminished product quality in assembly.
There are currently laboratory methods and in-plant audit techniques of measuring clamp load or bolt tension, but there is no acceptable procedure for on-line monitoring of these values. Implementation of clamp load control in assembly plants therefore is based on indirect methods which measure a quantity that correlates to tension, thus deducing the latter based on its empirical relationship to the former. There are two quantities that can readily be measured externally with instruments contained within the power tool that tightens the fastener. These quantities are torque and angle.
A usual practice in tightening a threaded fastener to make a joint has been to tighten the fastener to within a tolerance bracketed by predetermined specifications or limits of a low torque and a high torque. Other methods have also existed, such as the "turn-of-the-nut" strategy, also known as "torque-turn", in which the fastener is turned to within a range bounded by a low angle and a high angle, usually measured from a predetermined reference or threshold torque. Another more sophisticated method has been to tighten the fastener to a desired tension based on torque and angle measurements, as described in U.S. Pat. No. Re. 31,569, "Tension Control of Fasteners".
The use of simultaneous torque specifications and angle specifications is also in use in some assembly plants. For example, torque control may be used in which a low torque limit and a high torque limit are established to take into account meeting the maximum tension, the minimum tension, and assembly process capability requirements. In the assembly process, torque varies between the high and low torque limits, and angle scatters naturally in a band within any given confidence level. Angle specifications, in terms of a low angle limit and a high angle limit which can be derived from data for any given joint, are specified and used simultaneously for automatic detection of abnormalities.
Similarly, where the main mode of control is a predetermined angle range from some reference torque, it has been customary to introduce torque specifications for quality control purposes. If a joint is within the angle range but not the torque range, or vice versa, it is rejected.
The simultaneous existence of torque specifications and angle specifications defines a window on a plot of torque versus angle which is bounded on the top by the high torque limit, on the bottom by the low torque limit, at the left by the low angle limit and at the right by the high angle limit. A properly specified window defines a natural area on a plot of torque versus angle which encompasses torque-angle values that accomplish clamp load control and quality control goals in the assembly process. Such windows may or may not be rectangular or even closed.
Past attempts at reaching the window have generally been confined to discontinuing tightening as soon as the window is entered, i.e., when both the low angle limit and the low torque limit have been equalled or exceeded, based on measuring the torque and angle during the tightening process. These attempts have resulted in the final torque-angle value being less than optimal, and in some cases very close to the boundary of the window, resulting in a bias in these processes to produce a low clamp load.